Field of the Invention
The present invention pertains to an offshore or submarine oil and gas well support having a wellhead at the sea floor to which is connected an elongated riser which is supported by a buoyant tower having a superstructure extending slightly above the surface of the sea and disposed around the well flow control valves and associated components.
The development of offshore wells for hydrocarbon reservoirs and the like is complicated by the cost of providing and maintaining a total subsea completion system including the wellhead and the associated control valves and related components making up what is sometimes known as the Christmas tree. Although high production wells in moderate depths of from 300 to 800 feet may justify the expense of total subsea completion systems, the expense and hazards associated with servicing these wells is also an important consideration.
Alternatively, particularly in shallow waters, single well completion systems usually include a fixed support structure resting on or anchored to the seabed and supporting above the surface the wellhead and the well flow control and servicing components. However, for relatively low production wells the cost of the support structure may not justify completion of the well. Accordingly, there has been a need for an offshore well support system, particularly adapted for single wells, which can be considered for wells of a wide range of productive capacity, but which can more easily justify the completion of and production from wells which would otherwise not be considered. In particular, there is a need for a cost effective well support and production system for wells in depths greater than about 300 feet.
Certain considerations for relatively low production wells or reservoirs include the ease of installation of the well support structure, its ability to accommodate various sea states and the ease of access to wellhead flow control and flow monitoring components for servicing and workover of the well. In this latter regard, it has been considered important to provide a support structure which supports the Christmas tree only slightly above the sea surface so that a conventional workboat may be used for minor maintenance and wireline operations and certain types of floating drilling rigs, such as semi-submersible rigs, may be moved into position over the well while operating at their transit depth without disassembling major components of the well support structure. These problems and desiderata associated with the development and construction of certain offshore well support systems have been solved and satisfied, respectively, by the present invention.